The present invention relates to a time division multiplex (TDM) communication system and, in particular, to a frame phase alignment for use in a system.
In the TDM communication system, a plurality of data signals or channel signals are time-division multiplexed with each other to form a frame signal together with a transport overhead signal for controlling transportation of the data signals in the frame signal. That is, a frame format of the frame signal consists of a plurality of time slots. Some of the time slots are assigned for carrying the transport overhead signal and are called an overhead portion. The remaining time slots are assigned for carrying the data signals, respectively, and are called a subframe portion.
The data signals have different channel numbers and are assigned to the time slots in the subframe portion in the order of the channel numbers. Therefore, when a particular one of the time slots is indicated for carrying a leading one of the data signals, the remaining time slots are automatically determined for individually carrying the remaining data signals.
In order to indicate the position of the particular time slot in the frame signal, the overhead portion has information of the number of time slots from the overhead portion to the particular time slot which information is called a message pointer or a data pointer.
In the TDM communication system, a plurality of TDM signals are required to be synchronized with each other in order to, for example, perform exchange or switching between time slots in those TDM signals. To this end, a frame aligner is conventionally used in the TDM communication system.
A known frame aligner comprises a buffer memory in which an input frame signal is stored and is then read from the buffer memory under control of a frame synchronous signal on an output side, which will be referred to as an output frame synchronous signal, so that the input frame signal is phase shifted and is reproduced with a different frame phase synchronized with the output frame synchronous signal as an output frame signal. Thus, a plurality of TDM signals are synchronized with each other by use of frame aligners using a common output frame synchronous signal.
However, the known frame aligner has a disadvantage that the buffer memory has a memory capacity sufficient to store are of a single frame signal. Furthermore, when a large phase difference is between the input and the output frame signals, a large delay is caused at the buffer memory and therefore results in degradation of the signal in performance.
The TDM communication system sometimes deals with frame signals which are different from or asynchronous with each other in bit rates or clocks. In order to perform the frame phase alignment of those different TDM signals, it is required that those TDM signals are previously matched to each other in the bit rate.